We plan to extend behavioral and cellular techniques developed in the study of simple defensive behaviors and simple non-associative learning in the Aplysia to a more complex defensive behavior, escape locomotion, and a more advanced form of learning, associative conditioning. We focus on locomotion because it is central to the animal's behavioral repertory, and is modulated by hormones, by biogenic amines and by associative learning. We propose to use the control of locomotion in Aplysia as a model system for exploring the cellular mechanisms of four behavioral problems: The neural circuit underlying locomotion and the organization of the central program. The modulation of the central program for locomotion by hormones and by biogenic amines. The cellular mechanisms of associative learning. An analysis of what the animal learns during the acquisition of classical conditioning. Following earlier studies on sensitization and habituation, the present study also directs itself to the mechanistic relationships between non-associative and associative forms of learning.